If you're of the mindset that you charge adult working kids keep then I think it'd be very unfair not to charge a "part time" child who, nonetheless, maintained their own room in your home, accessed all the facilities, and partook of all the usual meals/snacks/drinks while they were there.
I wouldn't necessarily charge each adult child the exact sum - but I would charge them each the same percentage of their take home pay - and then pro-rata that for the kids who weren't there full time. I'm not sure why this should be considered such a terrible thing - as others have said, "part time" kids are either part of the family or they're not, and if they are, then they should be included in all aspects of family life, including the less desirable, less interesting, less fun stuff - like starting to take responsibility for yourself at both places you see as home.
I don't think this is something for the parents to discuss and/or negotiate between them either - not when the "child" is now an adult. It's the adult child themselves who should be negotiating with each parent - and of course each respective parent may have different ideas about what they think's appropriate - it doesn't have to be the same in each household (e.g. some would want a percentage, some would want a set specific sum, some would want regular chores performed in lieu of keep and so on). Any decent parent - whether they're the "main" household or not shouldn't see anything wrong in principle with the pro-rata idea (assuming they want to charge keep at all) - so the adult child shouldn't lose out or have to pay double or anything. I'd think a "main" parent who insisted on a full week's keep despite their child spending 2-3 days and nights elsewhere every week would be very wrong.
Finally, I think it's important for all the adult children, however many days they live in a household, to feel that they're each making a fair and equal contribution to the running of that home. Given the potential disparity in income, that doesn't have to mean they all pay £75 a week for example, but if they all pay the same % of take home, pro-rataed if necessary, then there can't be any dispute. If "part time" kids aren't asked to contribute I can imagine that that'd cause a huge amount of resentment and ill feeling from "full time" kids - they might well take the view that if their step siblings are allowed, say, 3 days for "free" that they should also be granted the same privilege.